Wide band interstage coupling network



April 1947- w. E. BRADLEY 2,419,882

WIDE BAND INTERSTAGE COUPLING NETWORK Filed Jan. 10, 1944 IN V EN TOR.

a 14:3 m 8 a Patented Apr. 29, 1947 WIDE BAND INTERSTAGE COUPLINGNETWORK William E. Bradley, Swarthmore, Pa., assignor, by mesneassignments, to Philco Corporation, Philadelphia, Pa.,' a corporation ofPennsylvania Application January 10, 1944, Serial No. 517,650

11 Claims.

The present invention relates to amplifier circuits, and moreparticularly to amplifier circuits such as are suitable for operation atrelatively high frequencies in the range of ten to two hundredmegacycles, and for amplifying relatively wide frequency bands of theorder of five to twenty megacycles.

Heretofore numerous wide frequency band amplifiers have been devisedsuch as those commonly utilized in television receivers. The interstagecoupling networks ordinarily used consist of two tuned, circuits withreactive coupling provided by a 1r or T network of reactances joiningthe two circuits or by mutual inductive cou pling, which is equivalentto an inductance T network.

Such amplifiers are adequate for present day television receivers, butwhen much wider bands are required the above mentioned arrangements areunnecessarily complicated mechanically, difficult of realization, ordifiicult to construct and align in mass production.

It is, therefore, desirable to provide a wideband inter-stage couplingnetwork requiring less mechanical accuracy in its construction andwhich, for that reason, is better adapted to mass production methods. Inaccordance with the present invention, that end is achieved bysubstituting for the plural coupling inductances heretofore employed, asingle inductance having a high ratio of length to diameter and havingtwo intermediate taps which preferably are connected respectively to theplate and control grid of the vacuum tubes which are to be coupled,which inductance is tuned by the electrode-togro'und capacity of theintercoupled vacuum tubes to the band of frequencies which it isintended to transmit.

One of the primary objects of this invention is to provide a wide-bandinterstage coupling network which is well adapted to economical massproduction.

Another object is to provide a coupling network of the above indicatedcharacter wherein necessary tuning adjustments can easily and quickly bemade.

The invention itself is set forth in particularity in the appendedclaims, and other objects and advantages will become apparent byreferring to the following description taken in connection with theaccompanying drawing, wherem- Figure 1 is a circuit diagram illustratingan interstage coupling network for a wide band high frequency amplifierconstructed in accordance with the present invention; and

Figure 2 is a cross-sectional View through an inductive device such asused in the circuit of Figure 1 which, furthermore, has been providedwith suitable adjusting means for modifying the inductive efi'ect of thecoupling unit.

Referring to the drawing, it will be seen that in Figure 1 there areshown two vacuum tubes, II and 12, of a wide band amplifier arranged tooperate over a particular band in the vicinity of ten to one hundredmegacycles. Since it is quite customary to utilize vacuum tubes of themultigrid type in such circuits, the vacuum tubes H and I2 have beenshown to be of such type, although it is to be understood that theinvention is not to be limited to the use of these tubes or any tubes ofany particular number of grids or control elements. The vacuum tube Hhas an anode 13 which, together with the connections thereto, has acertain capacity with respect to ground represented by the dotted linecapacitor l4. Anode voltage is supplied through a suitable couplingresistor l5 connected between the anode and a suitable source of power.The anode of the vacuum tube I! is capacitively coupled through acapacitor It to the interstage coupling network comprising a singleinductance H. The inductance I! is formed so as to have a relativelysmall diameter and a great length and, hence, the ratio between thelength and the diameter is relatively high. For example, in oneparticular embodiment the inductance had a diameter of inch and a lengthof 1% inches, and was composed of 45 turns tapped so as to providesections having 10, 20 and 15 turns, respectively.

The vacuum tube [2 is provided with a control grid which, together withits associated connections, has a certain capacity with respect toground, indicated by the dotted line capacitor l8. The interstagecoupling inductance I1 is ar ranged with two taps l9 and 2D, the firsttap being connected to the capacitor l6 and the latter tap beingconnected to the control grid of the vacuum tube 12. Each end of theinductance l! is preferably connected directly to ground as shown. Suchan arrangement operates in a manner analogous to a double tunedtransformer, but may conveniently have a larger band width transmissioncharacteristic than those commonly employed for television receivers.

It furthermore will be appreciated that the interstage coupling networkthus provided has its frequency range determined by the total inductanceand the inductance of each section, the outer ends of which areeifectively in parallel with the electrode to ground capacities i4 andI8 of the associated vacuum tubes H and I2. This arrangement has a greatadvantage over two coils coupled by mutual inductance because, with thisarrangement, a high coefficient of coupling is easily and reproduciblymaintained with percentage band widths as high as 50%. If it is desiredto provide for inductive trimming without substantial disturbance of thecoupling, this may be accomplished in the manner illustrated in Figure2. The inductor I1 is wound on a suitable insulating form 2|, which maybe of any material having a good radio frequency power factorcharacteristic and low loss. If inductive trimming arrangements aredesired, the ends of the coil form support 2l' may be provided withcoaxial recesses for receiving either an adjustable powdered iron coremember or a brass or copper screw. To illustrate such an arrangement,one end of the coil supporting structure 2| has been shown as providedwith a recess 22 in which there is positioned an adjustable powderediron core member 23. In order to illustrate another manner of inductivetrimming, the other end of the coil support 2| has been provided with arecess 24 which is threaded so as to permit the adjustment of a copperor brass screw 25. It, of course, will be appreciated that in massproduction the tolerances permitted in the winding of the coil I! mayeither tend to be over or tend to be under, depending upon the type ofinductive trimming adjusting device provided.

Since in eiiect the accumulative mechanical errors of two or three coilsand their leads have been reduced to a relatively small possible errorin a single coil, it will readily be appreciated that with a large ratiobetween the length and the diameter of the coil II, it is possible tomanufacture this coil by mass production methods without incurring thedisadvantages of other prior art arrangements.

While for the purposes of explanation of the present invention, aparticular embodiment has been disclosed, it is, of course, to beunderstood that the invention is not limited thereto, since obviouslysuch modifications and alterations may be made, both in the circuitarrangement and in the structure, as may be commensurate with the spiritand scope of my invention as set forth in the appended claims.

I claim:

1. A radio frequency amplifier adapted for operation in a rangegenerally including the limits i of ten to two hundred megacycles,comprising a vacuum tube having a certain anode to ground capacity, asucceeding vacuum tube having a certain control grid to ground capacity,an inductive coupling means interconnecting said vacuum tubes, saidinductive coupling means together with said electrode to groundcapacities operating as a broad band amplifier device, said inductivecoupling means comprising a single in-- ductance coil having a highratio of length to diameter and being provided at intermediate pointswith two taps, means connecting the two ends of said coil to each otherand to the cathodes of said tubes, means connecting one tap to the anodeof said first tube, means connecting the other tap to the grid of saidsecond tube, and means for preventing unidirectional current frompassing between said anode and said grid.

2. In a high frequency amplifier, the combina-.

tion comprising a plurality of vacuum tubes and an interstage couplingnetwork interconnecting said vacuum tubes, said network comprising asingle winding inductor having a relatively high ratio of length todiameter, and means interconnecting each end portion of said inductor toan electrode of a difierent one of said vacuum tubes so as to beeffectively in parallel with the capacity existing between ground andsaid electrode.

3. A high frequency amplifier for operation at frequencies of the orderof or greater than ten megacycles, comprising a pair of vacuum tubesinterconnected by an interstage coupling network, said cqupling networkcomprising a single coil winding having a high ratio of length todiameter and being divided into three sections by two intermediate taps,means connecting each end of said coil to ground, means connecting oneof said taps to an electrode of one of said vacuum tubes, and meansconnecting the other of said taps to an electrode of the other tube,thereby to connect the end sections of said coil in parallel with thecapacity existing between said electrodes and ground.

4. A broad band amplifier adapted for operation at frequencies of theorder of or greater than ten megacycles, comprisin a plurality of vacuumtubes each having a certain capacity between its anode and ground andanother certain capacity between its control grid and ground, aninterstage coupling network interconnecting successive vacuum tubes,said network comprising a single inductive winding having a high ratioof length to diameter, means connecting each end of said winding toground, means connecting an intermediate point of said winding to theanode of the preceding tube, and means connecting another intermediatepoint of said winding to the control grid of the succeeding vacuum tube,each end portion of said winding being efiectively in parallel with thecapacity existin between the ground and the electrode with which saidend portion is associated, thereby to produce a broad band transmissioncharacteristic.

5. In a high frequency amplifier, an interstage coupling networkcomprising a single winding inductor having a relatively high ratio oflength to diameter and a plurality of taps whereby each end portion ofsaid inductor may be associated with an electrode of the vacuum tubeswhich said inductor is adapted to interconnect, and adjustable meanslocated at each end 0! said inductor for varying the inductive effect ofsaid network.

6. In a high frequency amplifier, the combination comprising a pluralityof vacuum tubes and an interstate couplin network interconnecting saidtubes, said network comprising a single winding inductor having arelatively high ratio of length to diameter and a plurality of tapsthereon to divide said inductor into three sections, each end section ofsaid inductor being connected to an electrode of a different one of saidvacuum tubes so as to be effectively in parallel with the. capacitybetween ground and said electrode, and means arranged at the end of saidinductor for varying the inductive eflect thereof, said means comprisinga longitudinally adjustable member of a material having a permeabilitydiffering appreciably from that of air.

7. The combination with a pair of cascaded amplifier tubes, of acoupling means interposed between said tubes and comprising aninductance coil having its ends connected to ground, and

having two intermediate taps connected respectively to an outputelectrode of the first tube and to an input electrode of the secondtube.

8. The combination with a pair of cascaded amplifier tubes, of acoupling means interposed betwen said tubes and comprising an inductancecoil having its ends connected to ground, and having two intermediatetaps connected respectively to an output electrode of the first tube andto an input electrode of the second tube, and means associated with saidcoil for varying the inductance thereof.

9. In a high frequency amplifier, a pair of vacuum tubes, an interstagecoupling network interconnecting said tubes, said network comprising asingle inductance coil, means connecting one portion of said coilbetween the output electrodes of the first tube, and means connecting adifferent portion of said coil between the input electrodes of thesecond tube, said coil portions being distinct from one another.

10. In a high frequency amplifier, a pair of vacuum tubes, a networkcoupling the output of the first of said tubes to the input of thesecond of said tubes, said network comprising an elongate continuousinductance coil having two end terminals and two spaced intermediatetaps, one of said end terminals and the adjacent inter- 6 mediate tapbeing connected respectively to the output electrodes of the first tube,the remaining end terminal and intermediate tap being connectedrespectively to the two input terminals of the second tube.

11. In a high frequency amplifier, a pair of vacuum tubes, a networkcoupling the output of the first of said tubes to the input of thesecond of said tubes, said network comprising an elongate continuousinductance coil having two end terminals and two spaced intermediatetaps, one of said end terminals being connected to the cathode of thefirst tube, the next adjacent intermediate tap being connected to theanode of the first tube, the remaining intermediate tap being connectedto the control grid of the second tube, and the other end terminal beingconnected to the cathode of the second tube.

WILLIAM E. BRADLEY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,990,781 Fitch Feb. 12, 19351,952,579 Black Mar. 27, 1934

